There is known a screen transfer system that distributes packetized screen information on a server terminal (server device) in real time from the server device to plural client terminals (client devices), which are connected to the server device through a network. The client device receives and displays screen information in a whole or part of a screen region possessed by the server device. Information on a user's manipulation performed by the client device is transmitted to the server device. The screen transfer system is expected to be applied to various uses such as a remote monitoring system because the screen information on the remote server device can be displayed and referred to by the device in front of the user.
In constructing the screen transfer system, there are two methods, namely, a method for packetizing contents of the whole screen region possessed by the server device at constant time intervals and distributing the contents to each client device, and a method for distributing only information on a difference between drawing states and information on the region where the change is generated every time the change in drawing state is generated in the screen region. In the difference distributing method, because only data of the necessary part is generated according to the change in drawing state, a network band can be efficiently utilized compared with the former method. In the latter method, there is an advantage that the number of drawing frames per unit time can be increased. Accordingly, a service constructed by the latter method is generally provided.
The information indicating the difference for acquiring the latest drawing state in the screen region is referred to as a screen update. The latest drawing state can be acquired by reflecting the screen update on a frame buffer in which the last drawing state is stored.
In the screen transfer system, it is necessary to achieve a balance between image quality and a responsive property. Particularly, in the case that the screen information acquired by the server device is transmitted to the client device without degradation in an environment having insufficient communication quality, such as communication between remote bases, a time necessary for the transmission is significantly lengthened. This is because the amount of data to be transmitted is large for a utilizable band width. As a result, a time (response time) since some sort of manipulation is performed in the client device until the screen change on the server device corresponding to the manipulation is reflected in the client device is lengthened. That is, the responsive property is degraded. On the other hand, the responsive property is improved when the screen information is subjected to compression processing to reduce the data amount of the transmission. However, user's visibility is impaired because a compression method such as JPEG is accompanied with the degradation of the image quality. Thus, usually there is a trade-off relationship between the image quality and the responsive property.
There is known a method for not impairing the operability and the visibility by selecting the image quality of the screen information according to a communication rate of an acquired communication channel.